An electric power system also known as an electrical grid, or grid, is a network of interconnected components that typically involve a generation element (such as a nuclear power plant, a hydro-electric plant, or a smaller plant powered by gas fueled turbo generators), a transmission system (high voltage lines), a distribution system (lower voltage lines), and a set of customers or loads (lowest voltage levels). The connection between elements of the grid is accomplished through a set of step-up transformers (e.g., increasing voltage from a generating station to the transmission line) and step-down transformers (e.g., decreasing voltage from transmission lines into the distribution system via a substation equipped with such transformers). A healthy operation of the entire grid depends on healthy operation of each element of the grid. This is important from a cost perspective as well as safety perspective, which in turn makes optimal operation of the grid an important aspect even for policy makers, which is exemplified by Stan M. Kaplan's report to Congress, “Electric Power Transmission: Background and Policy Issues,” CRS Report for Congress, 2009. The disclosure of each reference set forth herein is incorporated herein by reference in its entirety.
The health of the grid and its components is typically ensured by automatic generation control (AGC), which is a system for adjusting power output across multiple generators at different power plants and substations, especially in response to changes in consumption, or load. Since inception of electricity and power generation, this control was accomplished with fixing a single generating unit as a reference for regulation and having the remaining generating units be controlled using the so-called “speed droop”. According to Woodward Corp, “Speed Droop and Power Generation,” Woodward Application Note 01302, 1991, speed droop is a governor (or prime mover driving a synchronous generator) function which reduces the governor reference speed as fuel position (load) increases and vice versa. With modernization of power generation control, multiple generation units are enabled in regulation, which reduces wear and tear of each individual unit's control mechanisms and improves overall system operation and performance.
The disclosed invention is in the general field of automatic generation control, which is accomplished through two primary methods. The first method is the active power (P) and frequency (F) control, while the second method is reactive power (Q) and voltage (V) control. When connected to the grid, the frequency of a power plant network is fixed by the grid. The P/F control in this case is P control only. The purpose of P control is threefold. First, its purpose is to ensure optimal sharing of the load among the generators. Additionally, P control serves the purpose of maintaining the exchange of active power with grid in accordance to a defined set point. Finally, P control ensures adequate reserve capacity to maintain system stability in the event of an incident.
In cases where any sub-network operates in an island mode, the frequency of the sub-network is determined by the generators connected to the islanded part of the power plant network. In these cases the purpose of control has a distinct threefold purpose of controlling frequency at pre-defined set point by producing power set points (in MW) to all operating generators, ensuring optimal sharing of load among generators, and ensuring adequate reserve capacity to maintain system stability in the event of an incident.
Strictly speaking, active power and frequency control is not part of the presently claimed invention, however, it interacts with the claimed invention of reactive power and voltage control functionality and its overview is provided henceforth for completeness purposes only. Active (real) power and frequency control is known in the art with a good background provided by Chien-Ning Yu, “Real Power and Frequency Control of Large Electric Power Systems under Open Access,” Master's Thesis, MIT, 1996.
It is imperative to have proper active power and frequency controls in place in order to achieve optimal operation and avoid system collapse. It is known that frequency within a power grid is constant when the same amount of electrical power is produced as consumed by the loads, including system losses. This is the optimal system state, however, if this is not the case frequency changes will occur. For example, the frequency of the system is reduced when a load increase is not compensated for by a corresponding increase of the turbine power of the connected generators. The power deficit will then decelerate the generator rotors and consequently reduce the frequency. Frequency reductions may also arise when production is lost, e.g., as a consequence of failures in the system where various safety response mechanisms disconnect the failed equipment from the grid. Significant reductions in frequency could lead to system collapse, due to the fact that most power station equipment, e.g. power supply systems, does not tolerate abnormally low frequencies. On the other hand, a load reduction in the system which is not compensated for by a reduction of turbine power leads to frequency increases, which could also destabilize the entire system.
The reactive mechanism similar to P/F control is Q/V control. Precise voltage control is required to ensure correct operating conditions for generators and loads. Voltage control is directly related to production and distribution of reactive power. Reactive power is the power used to support the transfer of real or active power over transmission and distribution networks in alternating current (AC) power systems, which are the majority of systems in modern power generation. In other words, reactive power is a large part of the cost associated with power generation and is a metric of a grid's efficiency to provide power to customers. The reactive power output of generator is controlled by means of machine excitation. Also, since transformers do not produce or consume any reactive power (they actually absorb reactive power), the Q/V control algorithm has to operate on both generators and transformers. As described in Larsson U.S. Pat. No. 7,956,596 B2, transformer voltage control is accomplished by changing transformer tap positions, which changes the flow of reactive power through the transformer. The purpose of Q/V control is threefold. First, Q/V control provides voltage control of selected busbars. Additionally, Q/V control ensures proportional sharing of reactive power among generators and transformers. Finally, Q/V control serves to limit the exchange of reactive power with the grid within a pre-defined range.
It is important that the voltage deviations in the system are limited. This is of importance for the connected loads, but a “good” voltage profile is also essential for keeping the losses low and for utilizing the reactive reserves to establish a secure operation of the system. Voltage control is, as been pointed out earlier, a more local control than the frequency control. Uniqueness and novelty of the disclosed invention is in the methodology used to accomplish coordinated voltage control, while controlling the distribution of reactive power between substations and reducing of interaction between voltage control modules.